If it is using NTLM authentication then trying to set the user name and password has no effect as it will use the identity of the host user. So no, you can't specify a custom username and password.
So if you want to use another user you would have to create a host running under that user, however that opens up a can of worms as that user would have to be a member of the Windows Group that the other host users run under.
Edit The other suggestion that others have made is to use a custom binding, this is blog Yet on BizTalk Impersonation With WCF Adapters by Paolo Salvatori goes into detail how to achieve that.